1. Technical Field
The present invention generally relates to semiconductor fabrication and, in particular, to a system and method for optical measurement of planarized features.
2. Background Description
Process control is becoming very critical with escalating processing costs and large wafer size. Out-of-specification processes result in large yield losses and thereby affect fabrication productivity. Real-time measurements of key process parameters (e.g., etch rates) are sometimes very difficult due to nanometer scale dimensions or a lack of non-perturbing measurement techniques.
However, it is often possible to correlate incoming feature dimensions to process and, in such a case, correlations can be developed between in-situ measurements (e.g., trench opening) of every processed wafer and a process parameter like etch rate. These correlations, in turn, can be uses to predict process end points and thereby control processes. Currently available optical methods provide only off-time measurement capabilities for monitoring purposes and hence are unsuited for real-time control.
The problem solved by the present invention is different from a large body of work presented by Raymond et al. in the following: C. J. Raymond, in “Milestones and Future Directions in Applications of Optical Scatterometry”, Optical Metrology, Proc. SPIE CR72, pp. 147-177, 1999; C. J. Raymond, in “In-situ Metrology”, Handbook of Semiconductor Manufacturing Technology, edited by Nishi et al., published by Marcell Dekker, 2000; C. J. Raymond, in “Scatterometry for Semiconductor Metrology”, Handbook of Silicon Semiconductor Metrology, edited by A. Diebold, to be published by Marcell Dekker, 2001; Raymond et al., in “Resist and Etched Line Profile Characterization Using Scatterometry”, Integrated Circuit Metrology, Inspection and Process Control XI, Proc. SPIE 3050, 1997; Raymond et al., in “Metrology of Subwavelength Photoresist Gratings Using Optical Scatterometry”, Journal of Vacuum Science and Technology B, 13(4), pp. 1484-1495, 1995; Raymond et al., in “Multi-Parameter Grating Metrology Using Optical Scatterometry”, Journal of Vacuum Science and Technology B, B15(2), 1997. Raymond's work describes gratings with surface profiles. The features in the inventions described by Raymond have not been planarized and consequently have significant surface reliefs.
The problem solved by the present invention is also different from that described by Milner et al., in “Latent Image Exposure Monitor Using Scatterometry”, SPIE Vol. 1673, pp. 274-283, 1992. The preceding is directed to latent images in photoresists. These gratings do not have surface relief but are planar gratings. Primarily, the features mentioned in Milner's work are latent images in photo resist. The difference in the refractive indices in the constituent components of these phase gratings is very small. In addition, the difference in the refractive indices is limited to the resist layer as the underlying anti-reflection coatings are not affected by exposure to light.